Implantable reconfigurable bone adjustment devices are occasionally used in orthopedic procedures to gradually adjust the position, orientation, geometry and/or length of a bone, such as, for example, by distraction, compression, realignment or bone transport. One embodiment of an implantable reconfigurable bone adjustment device is a limb-lengthening nail (LLN) configured for implantation in the medullary canal of a long bone and subsequently manipulated to adjust the length of the bone. Another embodiment of an implantable reconfigurable bone adjustment device is a bone transport nail or rod configured for implantation in the medullary canal of a long bone and subsequently manipulated to move a middle bone fragment or segment across a gap between proximal and distal bone fragments to induce bone regeneration in the gap. Still other embodiments of implantable reconfigurable bone adjustment devices include spinal adjustment implants and implants configured to achieve other gradual adjustments to the shape, position or length of skeletal structures. As can be appreciated, the size of these devices that are implanted in the medullary canal may be limited by the size of the medullary canal which may also limit the strength of these devices.
Some embodiments of implantable reconfigurable bone adjustment devices include internal magnets that are configured to rotate upon actuation by an external actuating device, thereby driving a threaded rod that engages other device components to achieve a dimensional modification of the device or other relational modification between components of the device. Such dimensional modification or relational modification of the device operates on bone segments, portions or fragments to which the device is affixed to exert pressure on the bone segments, portions or fragments, thereby gradually moving the bone segments, portions or fragments relative to one another. Such devices include a first member or part configured to be affixed to a first bone segment, portion or fragment; a second member or part configured to be affixed to a second bone segment, portion or fragment; a rod with at least one thread, the rotation of which causes displacement of the second member or part relative to the first member or part, and a mechanism for controlling the rotation of the threaded rod.
In the case of certain LLN devices, for example, the second member or part is assembled telescopically relative to the first member or part and rotation of the threaded rod operates to telescopically displace the second member or part (which may be referred to as an inner body) relative to the first member or part (which may be referred to as an outer body), thereby increasing the distance between the bone segments, portions or fragments to which the first member or part and the second member or part are respectively affixed. In such LLN devices, the first member or part and/or the second member or part are configured to carry some amount of limb load and therefore may be prone to mechanical breakage for various reasons.
In use, rotation of the threaded rod may be driven by a component, referred to herein as a “drive mechanism,” whose actuation is controlled to achieve a desired amount of rotation over time and at a desired rate, to achieve a desired amount of bone adjustment at a desired rate. In certain devices, the drive mechanism includes a magnet hermetically sealed in a housing, although other types of drive mechanisms, such as electric motors, are contemplated. A common feature of such drive mechanisms, which may also include gear reducers, is that the threaded rod is rigidly affixed to a structure of the drive mechanism to achieve controlled rotation of the threaded rod. This structure is referred to herein as a “driver.” In such LLN devices, it is a delicate balance between the appropriate amount of mechanical advantage using the drive mechanism, the appropriate amount of strength from the torsion of the magnet, and the appropriate amount of strength from the LLN device itself along with the size requirements of the LLN devices. Often balancing all of these factors may lead to premature failure which results in the inability of the device to perform its intended bone adjustment action.
The threaded rod in such devices necessarily engages at least one component of the device (other than the driver) such that rotation of the threaded rod changes the relative positions of different device components. This at least one component (other than the driver) is typically an internally threaded block that is attached to the first member or part and configured to receive the threaded rod.
When LLN devices are implanted in a patient, there is a required amount of travel or movement of the outer body (e.g., first member or part) relative to the inner body (e.g., second member or part) that must be balanced against an acceptable amount of stress in the welded junction between the outer body and the threaded block. If the device is fully extended, then there is an increased possibility that the LLN device can fail under certain loading conditions.
One form of attachment of the internally threaded block to the first member or part includes welding. The rotation of the threaded rod imposes forces on the threaded rod, the internally threaded block, and the weld between the internally threaded block and the first member or part. Often the welded material is inherently weaker than the threaded block or the first member or part thereby causing a risk for failure at the weld junction if the implantable reconfigurable bone adjustment device is overloaded.
While currently available bone adjustment systems have produced excellent results, many of these devices exhibit one or more shortcomings or disadvantages that render the device susceptible to failure. For example, a problem that has been encountered is that the stress concentrations at the point where the threaded block is affixed to the first member or part can cause failure of the device at this junction. Failures of a device at this junction results in the inability of the device to perform its intended bone adjustment action.
For these reasons among others, a need remains for further improvements in this technological field. The present disclosure addresses this need.